1996 Hyundai Accent Smell and poor gas mileage

Good afternoon. I just had a tune up to my vehicle: spark plug and wire, oil change and filter, fuel filter, air filter, brake pads, and rotors about 2 months ago. Now everytime I am sitting in one spot for a little while I can smell gas fumes from the exhaust in the car through the vents. Also I have been getting poor gas mileage, before the tune up I could go about 334 miles on 1 tank of gas now I have been averaging about 300 miles on 1 tank. Today I have to fill up and I only have about 250 on 1 tank.
Do you know if my fuel filter compartment has to be presurized, coud that be the reason I smell exhaust and the reason I am getting poor gas mileage?